This invention relates to recovering ethylene from a polyethylene production process. In particular this invention relates to recovering ethylene from a polyethylene production process, converting the ethylene to ethanol, and converting the ethanol to olefin.
Ethylene is one of the most important raw materials for organic chemical production. The present world demand for ethylene is about 80 million tons per year, with 57% of this demand attributable to the polyethylene industry. The preparation of polyethylene typically requires a highly purified ethylene raw material. An ethylene monomer concentration of 99.9 mole % is typically required for polyethylene production.
High purity olefin monomer is required throughout most polyolefin polymerization processes. During the typical olefin polymerization process, olefin monomer is charged to a reactor where some of the olefin monomer reacts to form polyolefins. Unreacted olefin monomer is recycled back into the reactor. Impurities in the monomer, such as methane, ethane, butylenes, and hydrogen can build up in the recycle stream requiring that a percentage of the recycle stream be purged.
The purge stream contains a high percentage of valuable olefin monomer, and recovery of this monomer is important for both economical and environmental reasons. However, the separation of impurities from light olefins such as ethylene and propylene typically requires expensive cryogenic distillation techniques.
Alternative techniques that do not require cryogenic temperatures have been explored. Dembicki et al. U.S. Pat. No. 4,623,704 discloses the use of membranes to recover the ethylene enriched gas in a polyethylene process. Mehra et al. U.S. Pat. No. 5,681,908 discloses an absorption process for the recovery of monomers in olefin polymerization processes.
The U.S. Environmental Protection Agency currently has a program to develop a process for recovering ethylene without the need for low process temperatures and with higher selectivities for purge stream impurities than offered by polymeric membranes.
Therefore, a need exists, for recovery of olefin, e.g., ethylene and propylene, during the polyolefin manufacturing process in order to reduce the loss of olefins. In particular, it is highly desirable to recover olefins in a highly pure and cost effective manner.
In order to recover olefins from a purge stream in a polyolefin production process, this invention provides for hydrating olefins in a purge stream to produce alcohols. The alcohol, after being separated from impurities in the purge stream, can then be used to make olefin products.
In one embodiment, the invention provides a method of purifying an olefin containing purge stream. The method comprises reacting a purge stream containing olefin and impurities with water in the presence of a hydrating catalyst to produce an alcohol containing stream. The alcohol from the alcohol containing stream is contacted with an olefin forming catalyst to form an olefin product stream.
Preferably, the impurities in the purge stream comprise at least one compound selected from the group consisting of methane, ethane, butylenes, and hydrogen. Preferably, the hydrating catalyst is a supported phosphoric acid catalyst. Preferably, the olefin forming catalyst is a silicoaluminophosphate molecular sieve catalyst. Preferably, the alcohol containing stream contains ethanol or propanol. Preferably, the reaction of a purge stream containing olefin and impurities with water in the presence of a hydrating catalyst is performed at a temperature of 180xc2x0 C. to 300xc2x0 C. and a pressure of 350 psig to 1000 psig.
In another embodiment, the invention provides a method of producing olefins from oxygenates. The method comprises mixing an oxygenate feed stream with an alcohol containing stream, the alcohol containing stream produced from a purge stream in a polyolefin production process. The mixed oxygenate and alcohol feed stream is converted in the presence of an olefin forming catalyst into an olefin product stream.
This invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached FIGURE and the appended claims.